select count(distinct a.sid) from students a inner join sc b on a.sid=c.sid and c.cid<>2
select count(distinct a.sid) from students a inner join sc b on a.sid=b.sid and c.cid<>2 inner join
sc c on a.sid=c.sid and c.cid=3
select count(distinct a.sid) from students a inner join sc b on a.sid=b.sid and c.cid<>2 inner join
sc c on a.sid=c.sid and c.cid=3
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